PRNP and scrapie: Due to the lack of sufficient amount of mesenteric lymph node blocks from scrapie-infected animals and the routine use of retropharyngeal lymph nodes diagnostic works, paired retropharyngeal lymph nodes and abdominal hemal nodes from naturally and experimentally scrapie-infected sheep, representing all three scrapie susceptible Prnp genotypes (Table 3) were used for the epitope mapping study.